In blow forming, a sheet of formable material is heated to a temperature at which it can be stretched by a pressurized working gas against a heated forming surface. The hot sheet material is gripped at its edges adjacent to the mold or die surface and pressurized air, or other suitable working gas, is applied to one side of the sheet to push and stretch the other side into conformance with the forming surface. The sheet is thus permanently deformed and the gas is vented and the formed sheet product removed from the mold or die. The sheet material may, for example, be a suitable metal alloy, a synthetic polymer or the like.
Hot blow forming processes are used to form automotive body panels using an aluminum alloy, such as fine grained AA5083, in cold rolled and recrystallized sheet form. For example, the Rashid et al. U.S. Pat. No. 6,253,588, Quick Plastic Forming of Aluminum Alloy Sheet Metal, assigned to the assignee of this invention, describes such a process. The aluminum alloy sheet blank is heated to a suitable temperature in the range of about 400° C. to about 510° C. and stretched under the pressure of a working gas into conformance with the surface of a forming tool. The gas pressure is increased in a predetermined controlled pressure-time sequence (e.g., in stepwise increments) from ambient pressure to a final level in the range of about 250 psi to about 500 psi or higher. The strategy is to strain and shape the sheet metal as rapidly as possible without tearing or cracking it. However, the pressure application rate has necessarily been conservative because the workpiece heating mechanisms have not necessarily yielded precise temperature control from hour to hour or from workpiece to workpiece in continuous sheet metal forming operations.
Forming tools are often made of steel and are massive heat sinks. The tools may be heated by electrical resistance heating rods in a control circuit. When the tools are to be maintained at high temperatures, such as 400° C. to 500° C., effective maintenance of the target tool temperature often depends on balancing electric power input with the opening and closing of the tools to the ambient temperature on a regular time pattern. But in actual practice the temperature of the tools can vary by several degrees from a target temperature for many reasons. Restoring the actual temperature of a massive tool to a target temperature may require some time during which it is usually desired to continue efficient production of good parts on the tool.
It is an object of this invention to provide an improved method of coordinating working gas pressure application and sheet material flow behavior with actual forming tool temperature to improve the productivity of hot blow forming tooling while maintaining the quality of the formed sheet metal panel or other product.